Template

ABSTRACT

A template is formed from a layered structure comprising a substrate and a single-phase polymer layer positioned on the substrate. The polymer layer comprises a textured surface, the texturing being caused by induction of stress in the polymer layer. The template finds use in the manufacture of a structure on the nanometre scale, which comprises the steps of providing a template and molding a material on to the template, followed by removal of the molded material from the template to provide a structure on the nanometre scale, such as an array, a grid, an optical device or an electronic device. The template may be made by a method comprising the steps of depositing a layer of a single-, phase polymer on to a substrate, baking the resulting structure at a temperature below the glass transition temperature (T g ) of the single-phase polymer, texturing a surface of the polymer layer by inducing stress in the polymer layer and annealing the resulting structure to provide a template.

The present invention relates to a template for use in the manufactureof structures on the nanometre scale.

The provision of templates for use in the production of structures onthe nanometre scale and, in particular, the provision of templates toproduce very detailed and intricately patterned structures is verydifficult.

According to the present invention, a template is provided which isformed from a layered structure comprising a substrate and asingle-phase polymer layer positioned on the substrate, wherein thepolymer layer comprises a textured surface, the texturing being causedby induction of stress in the polymer layer.

According to the present invention, a method of manufacture of astructure on the nanometre scale comprises the steps of providing atemplate as defined above, molding a material on to the template andremoving the molded material from the template to provide the desiredstructure.

According to the present invention, a method of making a templatecomprises the steps of depositing a layer of a single-phase polymer onto a substrate, baking the resulting structure from the deposition stepat a temperature below the glass transition temperature (T_(g)) of thesingle-phase polymer, texturing a surface of the polymer layer byinducing stress in the polymer layer and annealing the resultingstructure from the stress-induction step to provide a template.

The present invention therefore surprisingly utilises the finestructures generated by topographic instabilities in single-phasepolymer films, and thus enables the production of highly intricate,organised structures on the nanometre scale, so-called “nanostructures”.

The method of making a template according to the present inventionprovides a simple, fast and effective way of producing a template, whichmay then be used in the production of nanostructures for use in avariety of applications. Patterning of the template may be controlled byoptimisation of the fabrication parameters, for example the temperatureor polymer film thickness employed.

The template of the invention may be used in the manufacture of avariety of nanostructures such as arrays, grids and electronic oroptical devices such as polarisers. Such structures have many,applications not only in the fields of optics and electronics but also,for instance, in molecular separation techniques, for example theseparation of DNA. Also, unlike processes which involve the use ofpatterned substrates, the method of manufacture of the invention doesnot employ lithography and therefore provides a new avenue for thefabrication of nanostructures.

The substrate comprised in the template of the invention is preferablyinorganic and more preferably comprises silicon. The thickness of thesubstrate will typically be approximately 0.5 mm.

Any single-phase polymer may be comprised in the template of invention,however, the single-phase polymer is preferably selected frompolymethylglutarimide (PMGI), polymethylmethacrylate (PMMA) andphotoresists, such as AZ5214E, which is manufactured by Clarland GmbHand comprises 2-methoxy-1-methylethylacetate as its main component. Morepreferably, the single-phase polymer is PMGI. The thickness of thesingle-phase polymer layer may vary depending on the intricacy of thedesired texturing or patterning of the template, however, it istypically in the range 50-300 nm.

The template may additionally comprise a thin, rigid layer comprising asemiconductor or a metal for example. This layer is positioned on thesingle-phase polymer layer and will typically have a thickness ofapproximately 10 nm. If the template comprises a semiconductor layer,the semiconductor will preferably be germanium, which is favourable forfurther pattern transformation.

In the method of making a template according to the invention, the layerof single-phase polymer may be deposited on to the substrate by anyconventional method such as coating, painting or spraying for example.The resulting structure is then baked at a temperature below the glasstransition temperature (T_(g)) of the single-phase polymer such that adegree of instability remains in the polymer to form a firm but flexiblefilm on top of the substrate. If a baking temperature of higher than theT_(g) of the polymer is employed, no instability remains in the polymer.If the single-phase polymer is PMGI, which has a T_(g) of approximately200° C., a temperature of less than 200° C. will therefore typically beemployed. Preferably the temperature of this baking step is in the range120-200° C.

A semiconductor layer may also be deposited on to the single-phasepolymer layer. In this embodiment of the method according to theinvention, the semiconductor layer may be deposited on to the polymerlayer by any conventional method such as sputtering. The semiconductorlayer is preferably applied to a structure comprising a substrate coatedwith a single-phase polymer layer which has preferably already beensubjected to a baking step at a temperature of below the T_(g) of thepolymer. Following deposition of the semiconductor layer on to thepolymer layer of such a structure, the resulting three-layer structureis then subjected to a further baking step again at a temperature ofbelow the T_(g) of the polymer layer.

A surface of the polymer layer is textured via induction of stress intothe polymer layer. The stress induced in the polymer is typically in therange 0.5-1 MPa.

The nature of the texture or pattern so-produced is highly dependent onthe applied stress, which can be applied such that highly organised andcomplicated patterns are achieved. For example, if a tensile orcompressive strength is applied, a lined pattern in the direction of thestress will be generated in the surface of the polymer layer.Preferably, stress-induction in the polymer layer results in theformation of parallel grooves in the surface of the polymer layer. Theseparallel grooves are created because, under stress, the formation ofwaves with a vector in the stress direction becomes energeticallyunfavourable thus producing periodically ordered structures in thesurface of the polymer layer. This idea is analogous to pulling awrinkled table cloth in opposite directions. The polymer film thusprovides a uniform striped pattern with a characteristic wavelength (λ)as the instability in the polymer layer is controlled by spinodaldewetting, ie. the dewetted wave structure is characterised by a singlewavelength.

One way in which stress may be induced in the polymer layer is via theuse of a load bearing member comprising at least one contact surfacewhich engages the surface to be textured. The load bearing memberemployed in this embodiment of the method of the invention may comprisepolydimethylsiloxane (PDMS), and typically has the shape of a truncatedprism. The contact surface of the load bearing member may be smooth ormay itself be textured.

The template of the invention is employed in the manufacture ofstructures on the nanometre scale, which are typically made frommaterials such as metals, alloys, ceramics and polymers.

The structures so-produced may include arrays, grids, electronic devicesand optical devices, such as polarisers. Of particular interest aremagnetic wire arrays, such as those comprising Permalloy (Ni₈₀Fe₂₀)which may be used in device applications.

The present invention will now be described with reference to thefollowing examples and to the accompanying drawings. In the drawings:

FIG. 1 is a side perspective view illustrating the stress-induction stepof the method of making a template according to the present invention,including an enlarged detail of a textured surface of the template ofthe invention;

FIG. 2 shows atomic force microscope (ASM) images of (A) a randomlytextured surface taken from a 150 nm thickness PMGI film followingbaking at 160° C., and (B) an ordered surface resulting fromstress-induction in a 250 nm thickness PMGI film following baking at160° C.;

FIG. 3 illustrates surface patterns induced by localised stress and theanalysis thereof. (A) shows a surface structure obtained by pressing asample surface using a PDMS load bearing member which is patterned with20 μm square anti-dot patterns; (B) is a schematic illustration of thelocal stress distribution in sample A in which, for simplicity, onlyimportant stress components, τ are shown; (C) shows a defect-inducedstructure ordering; (D) illustrates the distance dependence of thewavelength in the vicinity of the defect;

FIG. 4 shows modulated wire patterns obtained by surface waveinterference, as follows: (A) a uniform pattern (φ₁) aligned at 160° C.;(B) a double-line pattern observed after heating sample with structureshown in (A) for 10 min at 205° C.; and (C) a single/double-linemodulated pattern obtained after heating the sample shown in (A) to 190°C. for 10 min.

FIG. 5 shows scanning electron miscroscopy (SEM) images of thefabricated structures and magnetization reversal measurement of thesuperalloy wires, as follows: (A) and (B) are two PMGI polymerstructures (random and aligned, respectively) defined by sequentialplasma etching, in which nanochannels were etched to the siliconsubstrate; (C) shows a Permalloy wire array obtained by lift-off; (D)illustrates magnetic hysterisis loops measured on 400 nm width and 30 nmthick Permalloy wire arrays, in which loop 1 was taken from anunpatterned film and loops 2 and 3 were taken when the magnetic fieldwas applied along and perpendicular to the wire axis respectively.

EXAMPLE 1 Formation of a Template Using a Load Member with a SmoothContact Surface

250 nm and 150 nm thick layers of PMGI (Micro Chem Corp., PMGI SF6) werespin-coated separately on to silicon substrates and baked at 170° C. for30 min. Then 10 nm-thick germanium was deposited on to the PMGI layersby sputtering. Random wave patterns were observed when heating thesamples above 130° C., which is well below the T_(g) of pure PMGI(approximately 200° C.).

A PDMS elastic truncated prism with a smooth contact surface was pressedon to each sample surface as shown in FIG. 1. This Figure shows thatwhen pressure was applied to the PDMS prism, the intended lateralexpansion of the PDMS prism generated a stress along the film plane andrendered the assembled surface structure ordered (panel O), while on thefree sample surface random wave patterns were formed (panel R).

The atomic force microscope (AFM) images of the two sample surfacesafter heating at 160° C. for 25 min are shown in FIG. 2. FIG. 2A shows a150 nm thickness film with a free surface, which comprises random waves,while in the case of an applied load to the 250 nm thickness film, thewaves are well ordered as shown in FIG. 2B. The area of the orderedstructure can extend over the whole sample (centimetre scale) withmillimetre size domains induced by non-uniform deformation of the PDMSprism.

In this example, the applied load was 0.5-1 MPa. A similar order oflateral expansion stress within the sample surface is expected becauseof the high Poisson's ratio of the PDMS. The mechanism of wave formationis based on the stress assisted dewetting of the polymer film involved,which is fundamentally different from those of other observed wavestructures, such as mechanical compression induced surface buckles.After removal of the applied load the sample was annealed at 160° C. forten hours and the ordered structure remained stable.

EXAMPLE 2 Formation of a Template Using a Load Member with a PatternedContact Surface

A load member comprising a patterned contact surface was formed bycasting PDMS against a 1.5 μm thick patterned photoresist layer. Theresulting PDMS structure was cut into a rectangular shape to provide aPDMS load member patterned with a 20 μm square anti-dot pattern.

This member was pressed into a germanium-capped PMGI film at 160° C. for25 min. As the PMGI film was elastic, there were clear traces of thePDMS patterns printed on the sample surfaces, as indicated by the letterP in FIG. 3A. In addition to these patterns, a new set of squarepatterns (as indicated by P′) was formed, which appeared as a copy ofthe initial PDMS pattern.

This additional formed patterning may be explained as follows. When thePDMS was compressed on the sample surface, the regions between holesstarted to expand as shown in FIG. 3B. The five typical expanding parts(the centre and four arms of a cross as indicated) generated acompressive strain in a square-framed region thus aligning the patternsalong the frame. The asymmetry of the alignment of ripples is attributedto the existence of an off-normal force applied to the PDMS, whichgenerates a tension along the horizontal direction, as shown by the openarrow in FIG. 3B.

In general, the value of applied stress is expected to be much smallerthan the internal stress of a film, which is responsible for the filminstability. The external stress is used merely to suppress thestructural disorder induced by thermal fluctuation and to align thewavelike patterns. The internal stress, which causes film instability,is accumulated due to the temperature rise during annealing and can beexpressed as: $\begin{matrix}{\sigma_{0} = {\int_{T_{0}}^{T}{\frac{E_{c}}{1 - v_{c}}\left( {\alpha_{p} - \alpha_{c}} \right)\quad{\mathbb{d}T}}}} & (1)\end{matrix}$where T₀ and T are, respectively, the stress free temperature and thetemperature to which the film is heated, E_(c) is the Young's modulusand ν_(c) the Poission's ratio of the germanium film, and α_(c) (α_(p))is the thermal expansion coefficient of the polymer film. For a PMGIfilm without a germanium capping layer no instability is found and thesubstrate effect can therefore be neglected. It is difficult tocalculate the value of σ₀ precisely since the value of α_(p) dependsstrongly on the temperature and an additional polymerized layer couldform at the interface between the polymer and the capping (germanium)layers. However, a reasonable estimate gives σ₀ of approximately 100MPa, based on E_(c)/(1−v_(c))˜10¹¹ Pa and (α_(p)−α_(c)) (T−T_(o))˜10⁻³.This is about two orders higher than the applied stress. Thus, theapplied stress only acts as a small perturbation to the isotropicinternal stress σ₀ and introduces an anisotropy which leads thestructure to order.

This can be further understood through the examination of the orderingof a local structure generated by a defect centre. FIG. 3C shows atypical structure at the vicinity of a defect on a load free sample.When a defect, for example a dust particle or pin hole, exists in apolymer film restrained by a capping (germanium) layer, the break offilm continuity leads to a redistribution of stress inside the film. Byexpressing the radial and traverse components of the stress around thedefect as σ_(r) and σ_(t), respectively, this gives:σ_(r)=σ_(o)(1−e ^(−r/ξ)),  (2a)σ_(t)=σ_(o)(1−v _(c) e ^(−r/ξ)),  (2b)where r is the radius calculated from the edge of the defect and ξ is acharacteristic length of the stress distribution. For stress-assistedinstability in a rubber-like polymer film, the relationship between thesurface wavelength and stress is λ=K/σ², where K is a constant.Considering that the redistribution of material during formation of thewavelike structure is caused by the internal stress along the wavevector direction, it follows that: $\begin{matrix}{\lambda = \frac{\lambda_{o}}{\left( {1 - {\nu_{o}{\mathbb{e}}^{{- r}/\zeta}}} \right)^{2}}} & (3)\end{matrix}$where λ₀ is the wave length of the structure far away from the defectcentre. Taking ν_(c)=0.4, the characteristic length ξ was found to beabout 10 μm by fitting equation (3) with experimental results as shownin FIG. 3D. If the radius of the whole ordered region is taken to be 20μm (see FIG. 3C), a value of the stress anisotropy required for orderingthe structures in a sample from equation (2) may be obtained as follows:$\begin{matrix}{\alpha = {\frac{\sigma_{t} - \sigma_{r}}{\sigma_{t} + \sigma_{r}} \sim {4\%}}} & (4)\end{matrix}$This result confirms that a small perturbation in the stress candramatically modify the structure morphology.

EXAMPLE 3 Provision of Complex Patterning Via Changes in ExperimentalConditions

This Example provides another method of making a template, the so-called“surface wave interference”, to create more complex patterns. Thewavelength of surface patterns is normally determined by the fastestgrowing wave mode in the system and strongly depends on experimentalparameters. If a wave pattern Φ₁ε_(1(t)e) ^(iq) ¹ ^(x) is thecharacteristic mode in a given experimental condition, a rapid change ofthe sample condition will create a new characteristic waveΦ₂=ε₂(t)e^(i(q) ² ^(x+φ)). In the time period when the decaying wave Φ₁and arising wave Φ₂ co-exist a new pattern induced by their interferenceis observed.

FIG. 4A shows an aligned wave Φ₁ created at 160° C. and FIG. 4B shows adouble line pattern obtained after further heating the sample for 10 minat 205° C. without the application of a load. This example shows thatthe dominant surface wavelength of the film at 205° C. is about twice ofthat at 160° C. (q₂˜q₁/2) due to strong softening of the polymer nearits glass transition point. FIG. 4B illustrates the pattern formed in afilm which has not yet reached its steady state. This may be expressedas Φ=Φ₁+Φ₂=ε₁(t)e^(iq) ¹ ^(x)+ε₂(t)e^(i(q) ¹ ^(x/2+φ)). The value of thephase shift φ is required for pattern symmetry. Similarly, thewavelength obtained at 190° C. is about 1.7 times of that obtained at160° C. After heating the sample with wave Φ₁ to 190° C. for 10 min asingle/double line modulated structure can be found, as shown in FIG.4C, which agrees well with Φ′=Φ₁+Φ′₂=ε₁(t)e^(iq) ¹ ^(x)+ε′₂(t)e^(i(2q) ¹^(x/3))

In order to utilise such an interference effect to create complexpatterns, it would be ideal if the wavelengths of both Φ₁ and Φ₂ couldbe chosen as desired. There is no limit to the number of the waves whichmay be included, and the obtained wave (Φ₁+Φ₂) may further interferewith another wave Φ₃ to create more complex patterning, e.g.Φ=[(Φ₁+Φ₂)+Φ₃]+ . . . . Desired structures displaying abundant linearrangements with the appearance of bar-codes are possible. Suchobserved interference patterns and their evolution process are of use inthe fundamental study of dynamic processes of polymer diffusion andcreep, and wave mode selection due to film instability.

EXAMPLE 4 Fabrication of a Nanostructure

The wavelength of the lined patterns obtained in the above-describedgermanium-capped PMGI template was in the micron to submicron range, andtheir amplitude was around 20 nm.

A 40 nm thick PMMA (Micro Chem Corp. 950 PMMA A2) resist layer wasspin-coated on to the template surface and the resulting structure wasbaked at 160° C. for 5 min before being cooled to room temperature. Aglass wafer was employed to protect the surface flatness of the PMMAlayer. After partially removing the PMMA layer by oxygen (O₂) plasmaetching, the remaining PMMA in the trenches of the template was used asmask during etching of the thin germanium layer by sulphur hexafluoride(SF₆). Subsequently the patterned germanium layer was used as anothermask during etching through the PMGI by O₂ plasma. Finally, a layer offunctional material, such as metal, was deposited on to the structureand the desired nanostructures were obtained by lifting off the rest ofthe PMGI polymer.

By varying the parameters employed in the etching of the PMMA layer, theline width of the etched PMGI could be controlled. FIGS. 5A and 5B show,respectively, typical SEM images of random and ordered polymerstructures on a silicon substrate after the final reactive ion etching(RIE). The channel width obtained was approximately 150 nm and the wholepattern was uniform and defect-free over a large area.

FIG. 5C shows a magnetic wire array of 30 nm thick Permalloy (Ni₈₀Fe₂₀)obtained in this way. In recent years, such fine patterned magneticwires have attracted great scientific interest in particular in deviceapplications. The magnetization reversal of fabricated permalloy wireswere studied by the magneto-optic Kerr effect technique and the resultsare shown in FIG. 5D. Compared to the unpatterned film (loop 1), thelarge increase in the coercivity obtained with the field along the wire(loop 2) is attributed to the shape anisotropy induced complication ofmagnetization reversal, such as the so-called “bucking effect” etc. Whenthe field was applied perpendicular to the wires, a remarkable increasein the saturation field was observed (loop 3). This could be explainedby the “magnetic charges” induced along the wire edges, resulting amagnetically hard behaviour in the direction perpendicular to the wires.

1. A template formed from a layered structure comprising a substrate anda single-phase polymer layer positioned on the substrate, wherein thepolymer layer comprises a textured surface, the texturing being causedby induction of stress in the polymer layer.
 2. A template according toclaim 1, additionally comprising a semiconductor layer positioned on thepolymer layer.
 3. A template according to claim 1, wherein thesingle-phase polymer is selected from polymethylglutarimide (PMGI),polymethylmethacrylate (PMMA) and photoresist AZ5214E.
 4. A templateaccording to claim 2, wherein the semiconductor is germanium.
 5. Atemplate according to claim 1, wherein the substrate comprises silicon.6. A template according to claim 1, wherein the textured surfacecomprises parallel grooves.
 7. A template according to claim 1, whereinthe thickness of the single-phase polymer layer is 50-300 nm.
 8. Atemplate according to claim 2, wherein the thickness of thesemiconductor layer is approximately 10 nm.
 9. A method of manufactureof a structure on the nanometre scale comprising the steps of: providinga template as defined in claim 1; molding a material on to the template;and removing the molded material from the template to provide astructure on the nanometre scale.
 10. A method according to claim 9,wherein the structure is an array, a grid, an optical device or anelectronic device.
 11. A method according to claim 10, wherein theoptical device is a polariser.
 12. A method according to claim 10,wherein the array is a magnetic wire array.
 13. A method according toclaim 12, wherein the magnetic wire array comprises Permalloy.
 14. Amethod of making a template comprising the steps of: depositing a layerof a single-phase polymer on to a substrate; baking the resultingstructure from the deposition step at a temperature below the glasstransition temperature (T_(g)) of the single-phase polymer; texturing asurface of the polymer layer by inducing stress in the polymer layer;and annealing the resulting structure from the stress-induction step toprovide a template.
 15. A method according to claim 14 additionallycomprising the step of depositing a semiconductor layer on to thepolymer layer.
 16. A method according to claim 14, wherein thetemperature employed in the baking step is in the range 120-200° C. 17.A method according to claim 14, wherein the stress induced in thepolymer is in the range 0.5-1 MPa.
 18. A method according to claim 14,wherein stress is induced in the polymer layer using a load bearingmember comprising at least one contact surface engaging the surface tobe textured.
 19. A method according to claim 18, wherein the loadbearing member comprises polydimethylsiloxane (PDMS).
 20. A methodaccording to claim 18, wherein the contact surface of the load bearingmember is textured.
 21. A method according to claim 14, wherein thesingle-phase polymer is selected from PMGI, PMMA and photoresistAZ5214E.
 22. A method according to claim 15, wherein the semiconductoris germanium.
 23. A method according to claim 14, wherein the substratecomprises silicon.
 24. A method according to claim 14, whereinstress-induction in the polymer layer results in the formation ofparallel grooves in the surface of the polymer layer.
 25. A methodaccording to claim 14, wherein the thickness of the polymer layer is50-300 nm.
 26. A method according to claim 15, wherein the thickness ofthe semiconductor layer is approximately 10 nm.